build: update actions/checkout and actions/setup-python (#571)

Source-Link: https://github.com/googleapis/synthtool/commit/3551acd1261fd8f616cbfd054cda9bd6d6ac75f4
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-python:latest@sha256:230f7fe8a0d2ed81a519cfc15c6bb11c5b46b9fb449b8b1219b3771bcb520ad2

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>
2 files changed